This just isn't a good idea, and as said above probably not possible. As also said above, you would have the stress of worrying about exchange rate fluctuations. Besides, if you defaulted on your mortgage, how would the bank be able to repossess the house? Even if such a product was available, it would be prohibitively expensive.
You could of course secure a mortgage on a UK property and buy a house here for cash, but again your debt would be in Sterling and your assets and income in NZD.
Good luck with your house plans, though.